Accessories such as air compressors and alternators for automobiles are driven with a belt power-transmission system using an engine as the driving source. For the power transmission belt in the belt power-transmission system is utilized a V-ribbed belt, which contains a tension layer forming the back of the belt, a compression layer formed on one surface of the tension layer, having plural V-shaped ribs formed to extend in the perimeter direction of the belt and being a frictional power transmission surface against pulleys, and a tension member embedded between the tension layer and the compression layer to extend in the perimeter direction of the belt.
Such a power transmission belt has a problem that it generates noise when exposed to water. For example, in running in rain or the like case, when water penetrates into an engine room and the water further penetrates into the space between the frictional power transmission surface (power transmission surface) of the belt and pulleys, since the frictional power transmission surface is poorly wettable with water and tends to repel water, the water penetration condition could not be uniform between the frictional power transmission surface of the belt and the pulleys. With that, in the area where water does not penetrate, the friction coefficient does not lower and the belt is in close contact with the pulleys, while in the area where water has penetrated, the friction coefficient lowers. In that manner, areas of a dry state (DRY) and a wet state (WET) exist as mixed on the frictional power transmission surface, and if the difference in the friction coefficient between the two is large, stick-slip sound is often generated between the belt and the pulleys.
Recently, for power transmission belts, a demand of excellent quietness [quietness in a dry state (DRY) and in a watered state (in exposure to water, WET)] has become increasing. For such a demand, a technique of covering the frictional power transmission surface with a fabric has been proposed.
In Patent Document 1, a pulley contact side surface (frictional power transmission surface) of a belt body is integrally covered with a fabric layer, and a powder is made to exist in a state of infiltrating into the inside of the fibers of the fabric layer to thereby prevent rubber exposure so as to prevent slip sounding for a long period of time. This document discloses that a powder is sprayed onto the forming surface of a mold to form a pulley contact surface thereby forming a powder layer, the powder layer is pressed against a fabric layer wound around an unvulcanized rubber composition for belt formation (fabric layer laminated by being wound around a laminate of plural unvulcanized rubber sheets to form a back rubber layer, an adhesion rubber layer and a compression rubber layer), and the unvulcanized rubber composition is vulcanized to form a fabric layer with the powder penetrated into the frictional power transmission surface, thereby preventing the powder from dropping off.
However, in the embodiment of Patent Document 1, a powder layer must be formed on the forming surface of a mold, and therefore the molding and vulcanizing step is complicated and the mold is easily stained. Moreover, vulcanized rubber may readily bleed out on the surface of the fabric layer to cause stick-slip sound generation. Furthermore, the friction coefficient depends on the powder, and therefore even though the friction coefficient could be reduced in a dry state (DRY), the friction coefficient would be impaired in a wet state (WET). Patent Document 1 does not disclose any concrete data relating to sound generation.
Patent Document 2 discloses that a transmission belt having a metal soap layer formed by applying a powdery metal soap on the contact surface (frictional power transmission surface) thereof with pulleys can effectively reduce noise even in exposure to water. Patent Document 2 also describes that the inner peripheral surface (frictional power transmission surface) of the rib rubber layer is covered with a reinforcing fabric and a metal soap, and further describes that when a hydrophobic metal soap powder is applied, they hardly flow away by watering as compared with a layer formed of a hydrophilic powder such as talc, and the metal soap powder acts as a lubricant to reduce the friction between the belt and pulleys to thereby prevent noise generation during driving. Furthermore, it is disclosed that in Examples in Patent Document 2, the reinforcing fabric of the inner peripheral surface is formed of a polyester/cotton blended plain weave fabric and is subjected to an adhesive treatment with a RFL aqueous solution, and in Comparative Examples, though on the reinforcing fabric to cover the frictional power transmission surface is applied a powder of talc, silica, calcium carbonate or the like, the powder flows away with water and noise is generated in early stages.
However, in the transmission belt in Patent Document 2, since hydrophobicity is imparted to the frictional power transmission surface by a metal soap powder, the water penetration state is still non-uniform between the frictional power transmission surface and pulleys in exposure to water. Consequently, the difference between the friction coefficient in a dry state (DRY) and the friction coefficient in a wet state (WET) is large, which may cause slip sound generation.
Patent Document 3 discloses a V-ribbed belt in which the rub surface is covered with a canvas fabric that contains an elastic yarn and a non-elastic yarn and is stretchable in two predetermined directions for improving the durability of the rib surface and maintaining the rib surface state, the elastic yarn contains a polyurethane and the non-elastic yarn contains a cellulose-based fiber or yarn.
However, in the V-ribbed belt in Patent Document 3, rubber is made exposed out to the frictional power transmission surface by being made penetrate through the canvas fabric that covers the rib surface, and therefore the canvas part exposing the surface is small. Consequently, the friction coefficient in a dry state is high, and the friction coefficient in a wet state tends to lower.
Patent Document 4 discloses a power transmission belt, in which in the V-shaped compression part that engages with the V-groove of pulleys, short fibers aligned in the belt width direction are embedded in such a manner that the tips thereof protrude out from the side wall surface of the compression part, and a powdery viscosity reducer such as talc is attached as a state of sealing up the protruding part of the short fibers. Owing to the attachment of the viscosity reducer, the adhesiveness of the compression part surface (frictional power transmission surface) of the belt lowers (the friction coefficient thereof lowers), and therefore sound generation caused by friction can be effectively prevented.
However, in the power transmission belt in Patent Document 4, the inorganic powder drops off from the frictional power transmission surface in long-term driving, and therefore sound generation resistance could not be maintained.